The present invention relates to a time division multiple access data communication system in which a plurality of stations which shares a slotted communication channel through which message signals are transmitted.
The ALOHA system is the first random access multipoint packet data communication system. This system uses a single radio channel which is shared by a plurality of stations or data terminals. Whenever a station generates a packet, which is a message of a fixed length, in the ALOHA system, it transmits the packet on the common radio channel. Since more than one station may attempt to transmit a packet simultaneously, several transmissions may overlap. These overlapping transmissions are said to collide if any portion of two packets overlap. Whenever a collision occurs, random numbers are used to specify a period of time each conflicting station must wait before an attempt is made to gain access to the channel. To reduce increase channel utilization, the slotted ALOHA system was proposed in which the channel is partitioned into slots of time equal to a packet length and each station only transmits a packet at the beginning of a slot. In this way overlapping transmissions are forced to completely overlap. This technique substantially doubles the maximum channel utilization of the unslotted ALOHA system.
Since the slotted ALOHA system still operates on a random access basis, an increase in traffic causes collisions to increase with a resultant increase in retransmissions. Therefore, the total traffic increases disproportionately due to the retransmissions and prevents the channel utilization from reaching its maximum.
To reduce the effects of collisions in the slotted ALOHA system, a slot reservation scheme has been proposed. In this system, the channel is partitioned into frames each containing a reservation slot for transmitting a reservation packet and data slots for transmitting data packets. Each station transmits a reservation packet on a random access basis requesting slots as many as required for data packets to be transmitted. If the request is granted, data slots of a subsequent frame are assigned to the requesting station, which in turn transmits data packets on the assigned slots.
Because of the neccessity of the packet reservation system to transmit a reservation packet prior to the transmission of data packets, there is a delay before the data packets are actually sent. If short messages are dominant in the traffic, the amount of delay would become substantial and satisfactory channel utilization is not attained.